(1) Field of the Invention
The present invention relates to image display devices and methods of driving the same and, in particular, to an image display device using current-driven luminescence elements and a method of driving the same.
(2) Description of the Related Art
Image display devices using organic electro luminescence (EL) elements are well-known as image display devices using current-driven luminescence elements. An organic EL display device using such organic EL elements which spontaneously generate photons does not require backlights needed in a liquid crystal display, and is therefore ideally suited to achieving reduction of thickness of the devices. In addition, since the organic EL displays have an unrestricted viewing angle, the organic EL displays are expected to be put into practical use as a next-generation display device. Unlike liquid-crystal cells, which are controlled by a voltage applied to them, the luminance of each organic EL element used in the organic EL display devices is controlled by a current flowing through that element.
In organic EL display devices in general, organic EL elements constituting pixels are arranged in a matrix. An organic EL display device in which an organic EL element is provided at a crosspoint of each of a plurality of row electrodes (scanning lines) and each of a plurality of column electrodes (data lines) and a voltage corresponding to a data signal is applied between a selected row electrode and a plurality of column electrodes to drive the organic EL element is called a passive-matrix organic EL display device.
On the other hand, in an organic EL display device called an active-matrix organic EL display device, a switching thin film transistor (TFTs) is provided at a crosspoint of each of a plurality of scanning lines and each of a plurality of data lines, a gate of a driving element is connected to the switching TFT, and the switching TFT is turned on through a selected scanning line to input a data signal through a signal line into the driving element, which then drives an organic EL element.
Unlike the passive-matrix organic EL display device in which each organic EL element connected to each row electrode (scanning line) generates photons only in a period during which the row electrode (scanning line) is selected, the active-matrix organic EL display device can keep organic EL elements generating photons until the next scan (selection). Therefore the luminance of the display of the active-matrix organic EL display device does not decrease as the number of scanning lines increases. Accordingly, the active-matrix organic EL display device can be driven by a low voltage, achieving low power consumption. However, the active-matrix organic EL display suffers from luminance unevenness because different currents flow into the organic EL elements in individual pixels due to variations in characteristics of the driving transistors even when the same data signal is provided to the organic EL elements.
To address the problem, Japanese Unexamined Patent Application Publication No. 2008-122633 discloses a method of compensating for luminance unevenness caused by variations in characteristics of driving transistors. The method uses a simple pixel circuit to compensate for variations in characteristics among pixels.
FIG. 10 is a block diagram illustrating a configuration of the conventional image display device described in Japanese Unexamined Patent Application Publication No. 2008-122633. The image display device 500 illustrated in FIG. 10 includes a pixel array unit 502 and a driving unit which drives the pixel array unit 502. The pixel array unit 502 includes scanning lines 701 to 70m arranged in rows, signal lines 601 to 60n arranged in columns, pixels 501 in a matrix each of which is disposed at a crosspoint of each of the scanning lines and each of the signal lines, and power supply lines 801 to 80m arranged in rows. The driving unit includes a signal selector 503, a scanning line driving unit 504, and a power supply line driving unit 505.
The scanning line driving unit 504 supplies a control signal to the scanning lines 701 to 70m in sequence in a horizontal period (1H) to sequentially scan the pixels 501 row by row. The power supply line driving unit 505 supplies a power source voltage that switches between first and second voltages to the power supply lines 801 to 80m in synchronization with the line-sequential scan. The signal selector 503 selects one of a luminance signal voltage which represents a video signal and a reference voltage in synchronization with the line-sequential scan and supplies the selected voltage to the signal lines 601 to 60n in columns.
Here, two of the column signal lines 601 to 60n are disposed in each column; one of the two signal lines supplies the reference voltage and the signal voltage to the pixels 501 in odd-numbered rows and the other supplies the reference voltage and signal voltage to the pixels 501 in even-numbered rows.
FIG. 11 is a circuit diagram of a luminescent pixel of the conventional image display device described in Japanese Unexamined Patent Application Publication No. 2008-122633. FIG. 11 shows the pixel 501 in the first row in the first column. A scanning line 701, a power supply line 801, and signal lines 601 are provided for the pixel 501. One of the two signal lines 601 is connected to the pixel 501. The pixel 501 includes a switching transistor 511, a driving transistor 512, a storing capacitive element 513, and a luminescence element 514. A gate of the switching transistor 511 is connected to the scanning line 701, one of a source and drain of the switching transistor 511 is connected to the signal line 601, and the other is connected to a gate of the driving transistor 512. A source of the driving transistor 512 is connected to an anode of the luminescence element 514 and a drain of the driving transistor 512 is connected to the power supply line 801. The luminescence element 514 has a cathode connected to a ground line 515. The storing capacitive element 513 is connected to the source and gate of the driving transistor 512.
In the configuration described above, the power supply line driving unit 505 switches the power supply line 801 from a first voltage (high voltage) to a second voltage (low voltage) while the reference voltage is on the signal lines 601. While the reference voltages is also on the signal line 601, the scanning line driving unit 504 drives the voltage on the scanning line 701 to an “H” level to bring the switching transistor 511 into conduction, thereby applying the reference voltage to the gate of the driving transistor 512, and sets the voltage at the source of the driving transistor 512 to the second voltage, which is a reset voltage. With the operation described above, preparation for compensating for a threshold voltage Vth of the driving transistor 512 is completed. Then, the power supply line driving unit 505 switches the voltage on the power supply line 801 from the second voltage to the first voltage to cause the storing capacitive element 513 to store a voltage corresponding to the threshold voltage Vth of the driving transistor 512 during a correction period before a voltage on the signal line 601 is switched from the reference voltage to the signal voltage. The power supply line driving unit 505 then drives the voltage at the switching transistor 511 to the “H” level to cause the storing capacitive element 513 to store the signal voltage. That is, the signal voltage is added to the voltage corresponding to the threshold voltage Vth of the driving transistor 512 that has been previously stored and is stored in the storing capacitive element 513. The driving transistor 512 is supplied with a current through the power supply line 801 at the first voltage and provides a driving current equivalent to the stored voltage to the luminescence element 514.
In the operation described above, the two signal lines 601 are disposed in each column to increase the time period during which each signal line is at the reference voltage. In this way, a correction period for storing the voltage corresponding to the threshold voltage Vth of the driving transistor 512 in the storing capacitive element 513 is provided.
FIG. 12 is a timing chart of an operation of the image display device described in Japanese Unexamined Patent Application Publication No. 2008-122633. Shown in FIG. 12 are waveforms of signals on the scanning line 701 and the power supply line 801 in the first line, the scanning line 702 and the power supply line 802 in the second line, the scanning line 703 and power supply line 803 in the third line, a signal line assigned to pixels in an odd-numbered row, and a signal line assigned to pixels in an even-numbered row. A scanning signal to be applied to the scanning lines shifts from line to line in each horizontal period (1H). A scanning signal applied to one scanning line includes two pulses. The time width of the first pulse is longer, equal to or greater than 1H; the time width of the second pulse is smaller, a fraction of 1H. The first pulse corresponds to the threshold correction period described above and the second pulse corresponds to a signal voltage sampling period and a mobility correction period. Also a power supply pulse supplied onto the power supply line shifts from line to line in a 1H period. A signal voltage, on the other hand, is applied to each signal line once in 2H and therefore a period equal to or longer than 1H during which the signal line is at the reference voltage can be provided.
In the conventional image display device described in Japanese Unexamined Patent Application Publication No. 2008-122633, a threshold voltage correction period is provided as described above even when the threshold voltage Vth of the driving transistor 512 varies among pixels and therefore the variations are cancelled from one pixel to another to prevent luminance unevenness of an image.